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Community and Q&A

Sill slope? Sealant tape?

SouthGeorgianBay | Posted in General Questions on

How much slope is recommended for window sills? The house isn’t in an coastal or otherwise very exposed location, but isn’t in an arid or especially sheltered place either.

I’m planning on using beveled siding, placed on a flat plywood window buck, to help move water away from the opening; the exterior face of the window frame will be set about 4 – 5″ back from the outside of the exterior sheathing. I’m trying to figure out how much extra room (vertically) to include in the rough opening.

Has anyone had real trouble using sealant tape (Hannoband or Tremco, for example)? The window install will likely be during cool weather, hopefully slowing expansion down a little.

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  1. Expert Member

    Six %. Flash the sill, place the window on sloped shims to provide a drainage plane and provide a way for any accumulated water to make its way out beyond the WRB by either drilling holes in the bottom flange or mounting the window outboard of the rain screen.

  2. SouthGeorgianBay | | #2

    Thanks for the advice Malcolm. Good to know that even a gently slope will do the trick.

  3. GBA Editor
    Martin Holladay | | #3

    I'm not sure which Tremco product you are looking at, but your reference to cool weather leads me to guess that you are talking about Tremco ExoAir Trio (a type of gasket). If that's what you are talking about, you may be interested to read the product review here: New Green Building Products — June 2013.

  4. SouthGeorgianBay | | #4


    That is the product I was looking at - thanks for the link.

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